Amazon calculates a product’s star ratings using a machine learned model instead of a raw data average. The machine learned model takes into account factors including: the age of a review, helpfulness votes by customers and whether the reviews are from verified purchases.
I've been eyeing this book for some years but held off buying it. My role at the moment requires me to read stuff like this to work out our transformation. Finally bought it and wasn't disappointed. This is a good guide for people who have background with DBs and see what other options are out there. Integration, serialization, you name it, it's in the book!